Add contribution note to cops documentation generator

Occasionally I reviewed direct edit to cops documentation.

This is similar in intent to the following:
#11563

For source code comments, refer to the AsciiDoc documentation:
https://docs.asciidoctor.org/asciidoc/latest/comments/#comment-blocks

lib/rubocop/cops_documentation_generator.rb

@@ -251,9 +251,19 @@ def footer_for_department(department)
     "\ninclude::../partials/#{filename}[]\n"
   end
 
+  # rubocop:disable Metrics/MethodLength
   def print_cops_of_department(department)
     selected_cops = cops_of_department(department)
-    content = +"= #{department}\n"
+    content = +<<~HEADER
+      ////
+        Do NOT edit this file by hand directly, as it is automatically generated.
+
+        Please make any necessary changes to the cop documentation within the source files themselves.
+      ////
+
+      = #{department}
+
+    HEADER
     selected_cops.each { |cop| content << print_cop_with_doc(cop) }
     content << footer_for_department(department)
     file_name = "#{docs_path}/#{department_to_basename(department)}.adoc"
@@ -262,6 +272,7 @@ def print_cops_of_department(department)
       file.write("#{content.strip}\n")
     end
   end
+  # rubocop:enable Metrics/MethodLength
 
   def print_cop_with_doc(cop) # rubocop:todo Metrics/AbcSize, Metrics/MethodLength
     cop_config = config.for_cop(cop)